FIG. 1 is a circuit diagram showing the structure of a duplexer in the related art. It is common practice to provide a duplexer having the structure shown in FIG. 1 in a radio front-end unit of a mobile phone base station. The structure of the duplexer in the related art shown in FIG. 1 will be described.
Components of the duplexer shown in FIG. 1 will be described. The duplexer shown in FIG. 1 includes a transmission signal input terminal 110, an antenna terminal 120, a received signal output terminal 130, a transmission-band filter 140, and a reception-band filter 160.
A connection relationship between components of the duplexer shown in FIG. 1 will be described. The transmission signal input terminal 110 is connected to one terminal of the transmission-band filter 140. The antenna terminal 120 is connected to the other terminal of the transmission-band filter 140 and one terminal of the reception-band filter 160. The other terminal of the reception-band filter 160 is connected to the received signal output terminal 130.
It is known that the duplexer in the related art having the structure shown in FIG. 1 has a problem of being difficult to downsize. In other words, when the duplexer in the related art is downsized, distance between the transmission signal input terminal 110 and the antenna terminal 120, and the received signal output terminal 130 becomes short and spatial interference becomes great. As a result, a problem of the shortage of out-of-band attenuation amounts in both the transmission-band filter 140 and the reception-band filter 160 is known.
Concerning the above, Patent Literature (PTL) 1 (Japanese Patent Application Laid-Open No. 2003-124706) discloses a description related to a bandpass filter. This bandpass filter is a two-port circuit, which is at least logically bilaterally symmetric with respect to a symmetry plane dividing the right and left sides and which has one input/output port on each of the right and left sides, respectively. This bandpass filter has an upper-left circuit A1, an upper-right circuit A2, and a lower circuit B0. The upper-left circuit A1 has a port 1 located on the left side, a capacitance Ca with one end grounded, and a line A connecting this capacitance Ca and the port 1. The upper-right circuit A2 has a port 1′ located on the right side, a capacitance Ca with one end grounded, and a line A′ connecting this capacitance Ca and the port 1′. The lower circuit B0 has a ground circuit B1 located on the left side, a ground circuit B2 located on the right side, a ground circuit B3 located at the center, a line B connecting the ground circuit B1 and the ground circuit B3, and a line B′ connecting the ground circuit B2 and the ground circuit B3. The line A and the line B are so wired that at least parts thereof will be substantially parallel with each other to form a coupled line.
In other words, the bandpass filter described in PTL 1 is made up by forming filters at multiple stages with an amplifier placed between stages.